1. Field of the Invention
The present invention is related to a memory device, more particularly to structures and materials of a non-volatile phase-change memory device.
2. Description of Prior Art
Along with technology advancement, demands for a high quality non-volatile memory have been asked for. Except for high reliability, fast operation speed, high cyclability and large storage capacity become basic requirements. Accordingly, various types of non-volatile memories (NVM) for the next generation are emerging. These include but not limited to Magnetoresistive Random Access Memory (MRAM), Oxide-based Resistive Random Access Memory (RRAM), Ferroelectric Random Access Memory (FRAM), and Phase-Change Memory (PCM). Among them PCM utilizes a material whose phases can be changed rapidly and reversibly between an amorphous phase and a counterpart crystalline phase. Moreover, high electrical resistance of the amorphous phase and low electrical resistance of the crystalline phase provide high well-defined states, which can be made into two-bit non-volatile storage to multi-bits non-volatile storage.
Comparing to other technologies of the emerging non-volatile memories, operating speed, scalability, device reliability, process compliance, and manufacturing cost of the PCM make it with better competitiveness. Accordingly the Phase-Change Memory (PCM) is suitable for applications of high-density stand-alone or embedded memory. Since PCM has the capability of operation at 0.1˜20 ns, it is also potential to replace embedded Dynamic Random Access Memory (DRAM). If so, the high-speed and large capacity PCM is promising as the candidate of universal memory to replace DRAM and FLASH RAM as a whole. Especially, when the feature size is below 32 nanometers (nm) and critical dimension (CD) of a memory device is getting smaller, programming current needed by the PCM cells is also reduced. This is of great advantage for technical development of the Phase-Change Memory.
Conventional materials of the phase-change memory are chalcogenides especially that based on the ternary alloy system: germanium-antimony-tellurium (Ge—Sb—Te) has been the most widely studied. Within the ternary system, Ge2Sb2Te5 (GST) possesses an excellent material characteristic, which makes it widely applicable to phase-change compact discs and solid-state memory devices. Phase change between the amorphous and the crystalline phases of Ge2Sb2Te5 is very rapid. The resistance difference between the amorphous phase and the crystalline phase reaches five to six orders of magnitude. Crystallization temperature of the amorphous phase is about 160 to 180° C. and melting point is about 635° C.
However, Ge2Sb2Te5 (GST) still has drawbacks to be improved. The not high enough crystallization-temperature results in less thermal stability and the temperature for 10-year data retention (T10Y), being between 86° C. to 93° C., is less than a minimum requirement 100° C. Moreover, melting temperature of GST is rather high making the reset process more energy consuming. Besides, Te in the composition of GST is highly volatile to contaminate semiconductor process-equipment. Te-fume is toxic when exposing to the environment. Te is also very diffusive inside the device to deteriorate electrodes after prolonged operation of the device. This will give rise to a deviation of the original GST stoichiometry and the formation of holes to further decline the reliability and cyclability. Furthermore, a big volume change up to 9.5% occurs when phase changes between amorphous and crystalline phases of Ge2Sb2Te5. After long-term cyclability, the integrity of the film is damaged causing the film broken or even congealed into an island structure and even delamination. This further adds into the reliability issues. The biggest problem of all, from the mass-production point of view, Ge2Sb2Te5 and its modifications being multi-component are very difficult in precise and reproducible control of composition in manufacturing multi-millions devices a time on a large area base such as on 12-inch wafer.
Many companies and institutes are trying to improve the Ge2Sb2Te5 based memory devices. The improvement has been done by adding a fourth and even a fifth element, for instance, nitrogen (N), oxygen (O), silicon (Si), or an extra compound, for instance, silicon dioxide (SiO2). However, adding extra elements or compounds makes the composition even more complicated to worsen process control of device composition.
A binary alloy system excluding Te as a new material of the Phase-Change Memory has been proposed. Antimony (Sb) based binary alloy systems have been extensively explored due to their growth-dominate crystallization mechanism of amorphous phase. For instance the Si—Sb alloy appeared in Applied Physics Letters, Vol. 91, 222102, 2007, authored by T. Zhang, Z. Song, F. Wang, B. Liu, and S. Feng; and the Zn—Sb alloy, appeared in Japanese Journal of Applied Physics, Vol. 46, pp. L543-L545, 2007, authored by T. J. Park, D. H. Kim, S. J. Park, S. Y. Choi, S. M. Yoon, K. J. Choi, N. Y. Lee, and B. G. Yu; the Sn—Sb alloy, appeared in Applied Physics Letters, Vol. 95, 032105, 2009, authored by F. Rao, Z. Song, K. Ren, X. Li, L. Wu, W. Xi, and B. Liu; and the C—Sb alloy, appeared in IEEE Transactions on Magnetics, Vol. 47, pp. 645-647, 2011, authored by C. C. Chang, P. C. Chang, K. F. Kao, T. R. Yew, M. J. Tsai, and T. S. Chin. However, the aforementioned binary alloys cannot escape from a phase separation problem which means two crystal phases occurred in the amorphous phase during crystallization. This is a great challenge for the reliability of the memory devices. On the other hand, single element as the material of the Phase-Change Memory (PCM) can be an optimal choice. The patent application No. Tw 201138172 filed by the current inventors is one of breakthrough achievements of this technology.
Scientists have been studying whether single Antimony (Sb) element or single Bi (Bi) element can be the material of the Phase-Change Memory (PCM). Nevertheless, in the mid-20th century scientists discovered that as-prepared single element Antimony (Sb) film crystallizes spontaneously at room temperature unless the thickness is under a critical value. The value of critical thickness varies among reports due to the variation in each case the evaporation rate, substrate material, vacuum degree, or deposition method. In 1958, Palatnik and Kosevich discovered the critical Sb-film thickness of 15 to 25 nm on metallic or glass substrates, which appeared in Soviet Physics Doklady, Vol. 3, p. 818. In 1963, H. Horikoshi and N. Tamura discovered that when the evaporation rate is 35 nm/min the critical thickness of the Sb film deposited on a glass substrate is 50 nm, published in Japanese Journal of Applied Physics, Vol. 2, pp. 328-336. In 1976, A. Kinbara, M. Ohmura, and A. Kikuchi found the critical Sb-film thickness of 12.8 nm published in Thin Solid Films, Vol. 34, pp. 37-40. They also discovered that the crystallization temperature is greatly raised when the thickness of the film decreases from 100 nm to 10 nm; moreover, an equation was derived for the relationship between the crystallization temperature (Tc, in unit of Kelvin) and the thickness (d, in nm) That is Tc=T0+C/d wherein T0 and C are constants representing 250 K and 1 nm, respectively. In 1985, M. Hashimoto discovered that the thickness of a vacuum-deposited Sb film on a colloidion film is 11 to 16 nm; the critical thickness of a Bi film is 8 to 9 nm, published in Thin Solid Films, Vol. 130, pp. 171-180. However, above derived crystallization temperature of the amorphous films is just above room temperature, which is unable to be implemented to phase-change applications.
Scientists continuously researched into solving the problem of low crystallization temperature. The U.S. Pat. No. 7,807,989 B2 disclosed that under the critical thickness, the crystallization temperature of the Sb film doped with large amount of Oxygen and Nitrogen (about 30 atom percent, at %) is significantly raised. The cyclability of a device made thereof can reach dozens of times. The U.S. application Pub. No. 2009/0212274 disclosed a phase-change memory (PCM) made of pure Sb film whose thickness is controlled to be below 5 nm, the crystallization temperature can be raised, very much similar to the earlier study of Kinbara et. al., in 1976. Although the thickness is thinner in U.S. application Pub. No. 2009/0212274, the relation between crystallization temperature (Tc) and thickness holds as Tc=T0+C/d, with T0 and C representing 345 K and 250 nm, respectively. Moreover, in the US 2009/0212274, when the thickness of the Sb film is 5 nm, the Tc is only 30° C. (303 K). When the thickness is 4 nm, the Tc is 140° C. (413 K). Two issues soon arise. Tc of 4 nm pure Sb film is 20˜30° C. lower than that of GST hence is difficult in applying to the Phase-Change Memory (PCM) devices. Although Tc of 2 nm or 1.5 nm Sb-film is 210 and 240° C., respectively, being high enough for true application, the second issue soon arises. The sharp variation in Tc versus a subtle change in film thickness (such as 0.5 nm) brings about a difficulty in operation control of the memory device. And the process control to ensure a tolerance within, say 0.1 nm, is extremely difficult in particular the case of large area processing, 12 inch wafer for instance. It is hence a big challenge to keep the Sb film not too thin, for instance, at 5 nm or thicker, meanwhile, maintaining the Tc much higher than that of Ge2Sb2Te5 (160 to 180° C.).
The present invention aims to solve the following main technical problems of the existing PCM.
The conventional material of the PCM can be a ternary alloy or multi-component alloy systems, it is not easy to control stoichiometry of the alloy film in nano-sized device on a large area based manufacturing. And the thin single element film has a Tc too sensitive to a subtle variation in thickness below 4 nm. The present invention aimed to tackle both issues altogether.
The conventional GST material has a huge volume change of nearly 10% during phase change which results in delamination after repeatedly write/erase cycles. The present invention proposes a solution to the volume change issue by a suitable design of phase-change material and by additives to control the volume change to be less than 3%.
The conventional GST material of PCM encounters a substantial resistance drifting issue in the reset state. This gives rise to the reliability problem in multi-level memory. The present invention discloses preferable embodiments to show how to solve it.